1. Field of the Invention
The present application relates to charging circuits for charging batteries in portable devices.
2. Discussion of the Related Art
There are two categories of wireless chargers for portable devices. In this regard, the term “wireless charger” refers to a charger that does not connect to the device to be charged through a charging cable. In one category, which is referred to herein as “direct contact chargers”, a charger or charging station (“charge base”) transfers energy through direct contacts to the device being charged. In the other category, which is referred to herein as “inductive chargers”, a charger transfers energy over an electromagnetic field that couples the charger to the portable device being charged. Typically, an inductive charger is provided in the form of a charge base, and energy is transferred by inductive coupling from the electromagnetic field generated by the charge base to an electrical circuit, which in turns charges the batteries of the portable device.
An inductive charger typically has an induction coil which creates an alternating electromagnetic field from within the charge base. A second induction coil, provided in the portable device, takes power from the electromagnetic field and converts it into an electrical current to charge the battery. The two induction coils in proximity effectively form an electrical transformer. This form of induction charging has many disadvantages not present in direct contact charging. For example, relative to direct contact chargers, inductive chargers have a lower efficiency and increased resistive heating. As energy that is lost turns into heat, an inductive charger can get quite warm during charging. An increase in temperature unduly increases stress to the battery, so that batteries that are charged in this manner may not last as long, as compared to batteries charged on a mat or through a regular plug-in charger. The heat buildup, which occurs only during charging, represents a low efficiency that depends significantly on the relative position of the two inductively coupled coils. Implementations that use lower frequencies or older drive technologies charge more slowly and generate more heat. Unlike direct contact chargers, inductive chargers include drive electronics and coils, thus increasing complexity and manufacturing costs. Another disadvantage is a public health concern that the alternating electromagnetic field (˜5 W, at radio frequencies in the 80-300 kHz range) is typically used very close to the human body. Some charge bases transmit at 915 MHz, which is the frequency that is used for food heating in microwave ovens.
There are many ways to implement direct contact charging. One way uses point-to-point electrodes, such as those used in home cordless telephones. One disadvantage of point-to-point electrodes is device alignment (i.e., the charge base and the device being charged are required to be placed precisely aligned in position and in correct polarities). Another way uses multiple-point to strips, such as used in the Wildcharge system. The disadvantage is the device to be charged has to have multiple electrodes arranged in a small circle, which is usually provided at the weight center of the device to be charged to prevent tilting. If the device is tilted, electrical contact is lost and charging fails. Another disadvantage results from misaligned positions between the charge base and the device being charged (e.g., when two electrodes fall between two adjacent electrode strips).